As a method of transmitting a large volume of data at a high speed, a system combining an OFDM (Orthogonal Frequency Division Multiple) and CDMA is under study in recent years. As a system combining OFDM and CDMA, there are two schemes; a scheme whereby chips with spread data are mapped on subcarriers in a frequency domain and a scheme whereby chips with spread data are mapped on subcarriers in a time domain.
When data is spread in the frequency domain, there may be violent propagation channel variation in the frequency domain caused by frequency selective fading due to a multipath environment, which causes orthogonality among spreading codes to be lost deteriorating its reception characteristic though a frequency diversity effect can be obtained during despreading.
When data is spread in the time axis domain, a variation in the propagation channel in the time axis domain is relatively moderate compared to that in the frequency domain, and therefore little frequency diversity effect is obtained, but orthogonality among spreading codes is maintained. However, data assigned to subcarriers with sharp drops has a very low reception SNR, and therefore there is a high probability that the data may be completely erroneous.
Especially when codes are multiplexed using M-ary modulation such as 16 QAM, deterioration of reception performance due to loss of orthogonality among spreading codes is drastic, and therefore spreading in the time axis domain has a better characteristic than spreading in the frequency domain.